The majority of infant acute megakaryocytic leukemias (AMKL) in non-Down Syndrome children have a unique chromosomal translocation, t(1;22). This translocation fuses two novel genes believed to be transcription factors, OTT (a.k.a. RBM15) and MAL (a.k.a. MKL1/MRTF-A/BSAC), and results in the expression of a chimeric protein. Homologous structural motifs within the fusion have been identified in proteins known to be involved in differentiation, proliferation and leukemogenesis. To elucidate the pathophysiology induced by the OTT-MAL fusion protein, mice have been generated using a knock-in approach to express OTT-MAL from the OTT locus. By expressing the fusion protein from the endogenous OTT promoter, the stoichiometry and expression pattern of OTT-MAL found in the human leukemia should be reproduced. Evolving data suggests subtle differences in gene dosage and regulation have a critical impact on leukemogenesis and normal megakaryopoiesis. Identification of the pathways by which OTT-MAL may induce leukemogenesis will require a more comprehensive understanding of the component genes, OTT and MAL as their hematopoietic roles are unknown. Deletion of OTT or MAL in mice will provide an invaluable reagent in defining the pathways dysregulated by OTT-MAL in terms of gene expression and physiology. The role of OTT-MAL in t(1;22) AMKL will be investigated using routine models to address the following questions: Specific Aim 1. What are the consequences of OTT-MAL expression in mice? Specific Aim 2. What is the in vivo physiologic role of OTT and which pathways of endogenous OTT are utilized by the OTT-MAL fusion protein? Specific Aim 3. What role does MAL function have in normal physiology and in OTT-MAL-mediated dysregulation? The establishment and analysis of a murine model of t(1;22) AMKL will provide new opportunities for the study of pathogenesis in AMKL, potential therapeutic interventions and fundamental insight into megakaryocyte development.